In the electrophotographic or electrostatic recording processes, developers are used in forming electrostatic images or electrostatic latent images. The electrostatic images can be formed by two-component developers composed of a toner and carrier particles or by one-component developers composed of only a toner but no carrier particles. One-component developers include magnetic one-component developers containing magnetic powder and non-magnetic one-component developers containing no magnetic powder.
Recently, many core-shell structured toners have been developed. These toners have colorant-containing core layers and shell layers covering the core layers. The core-shell structured toners can, to a certain extent, balance heat displacement resistance, storage stability, and electrical charge stability etc. and achieve a better combination of properties. However, limited by the toner structure and/or its preparation, the current core-shell structured toners have many shortcomings and need to be improved. For instance, the current core-shell structured toners have single core-shell structures. In the preparation of a single core-shell structured toner, the sphericity is relatively difficult to control and it is very easy for sphere-shaped toner to form. Although sphere-shaped toners give high quality images of both good uniformity and color reproducibility, they have low friction force on the contacting points in the commonly used scraper cleaning system and therefore cause poor cleaning performance. The toner residue on the photoreceptor surface causes the printing quality to vary with the extension of development time. More particularly, for the all-color toners which contain red, blue, yellow and black colors, when sphere-shaped toner is used, it is even more difficult to improve the cleaning performance, especially for the scraper cleaning systems. Further, there are increased needs for diverse and personal printings and thus different printing machines have different requirements on the sphericity and particle sizes of the toners. The single core-shell structured toners cannot meet the increased needs due to their limitation in controlling the sphericity.